1. Field of the Invention
The present invention relates to an eyepiece lens and in particular to an eyepiece lens suitable for the use in a viewfinder of a single-lens-reflex camera.
2. Related Background Art
In a viewfinder of a single-lens-reflex camera, since a shooting lens serves as an objective of the viewfinder, it has such features that lens can easily be changed without generating parallax even in a telephotograph or a close-up, so that it has become a mainstream of a camera with interchangeable lens system.
A viewfinder of a single-lens-reflex camera is a real image viewfinder observing an enlarged image of a real image formed by a shooting lens through an eyepiece having positive refractive power. Generally, the viewfinder is constructed such that an inverted image formed by a shooting lens is erected by a quick return mirror and a roof pentagonal prism and an enlarged erecting image is observed through an eyepiece lens constructed by an achromatic cemented positive lens.
As gaining popularity of camera in senior people, the request for a viewfinder to be able to adjust its diopter becomes increasingly strong. An eyepiece lens constructed by three lens groups, in order from an eye point, a negative lens group, a positive lens group, and a negative lens group, diopter of which can be adjusted by moving the positive lens group has been known.
A conventional eyepiece lens capable of adjusting diopter has a drawback that magnification is low relative to that of an eyepiece with fixed diopter. Moreover, since the diameter of a pupil is only about 4 mm, when the pupil of a photographer is opened widely in a dark place, or when an eye of a photographer and exit pupil of an eyepiece lens are shifted with each other, appearance of the image in the viewfinder becomes severely deteriorated. In order to overcome the aforementioned defects, eyepiece lenses have been proposed in the following patent documents such as Japanese Patent Application Laid-Open Nos. 2000-98266 and 2001-100115.
However, in an eyepiece lens disclosed in Japanese Patent Application Laid-Open No. 2000-98266, the eyepiece lens using an aspherical lens made of resin has high optical performance at reasonable cost, but magnification is slightly low. Moreover, in an eyepiece lens disclosed in Japanese Patent Application Laid-Open No. 2001-100115, magnification is also slightly low and an aspherical lens made of a glass material is used, so that it becomes costly.